The present invention relates to carburetors for internal combustion engines of the kind comprising a fuel jet system opening into the intake pipe of the engine and comprising an emulsion well supplied with air and fuel and a device for correcting the fuel/air ratio of the mixture supplied to the engine as a function of the amplitude of the alternating pressure variations caused by the suction of the engine in the intake pipe.
By nature, the intake of a piston spark ignition engine is subjected to pulsating operating conditions due to the alternating movement of the pistons; the pulsating operating conditions are particularly noticeable in modern engines in which the closure of the inlet valves takes place with considerable delay; it is also very noticeable in engines having a small number of cylinders, typically flat-twin engines. When such a motor is under a high load, the throttle of the carburetor is widely open.
The pulsed depression is applied directly to the fuel jet system, which opens upstream of the throttle. The result is operating anomalies. Depending on the geometrical configuration of the carburetor-manifold assembly, the acoustical matching of which is not easily controllable, the air-fuel mixture supplied to the engine may be to lean or too rich.
Different remedies have been proposed for countering the pulsatory phenomenon effects. French patent No. 2,355,170 corresponding to U.S. Pat. No. 4,092,380, describes a carburetor in which the correcting device meters the vacuum prevailing above the fuel as a function of the alternating component of the depression which is applied thereto. That device gives satisfactory results, but is often costly to put into practice; the particular embodiment described in that prior patent requires a system of valves for subjecting a diaphragm to a differential pressure depending on the alternating component of the depression in the intake pipe.
It is an object of the invention to provide a carburetor having an improved correction device. It is a more particular object to provide a carburetor having such a device which is of low cost and whose correction may be easily adapted to suit the requirements of different types of engine.
According to the invention, there is provided a carburetor of the above defined type, in which the correcting device comprises a movable wall defining a chamber which communicates with the emulsion well and to which the alternating pressure variations are applied through attenuating means, whereby the device adjusts the depression in the emulsion well for increasing or reducing the amplitude of the alternating component of the pressure which is communicated thereto from the opening of the fuel jet system, according as to whether the acoustic matching of the carburetor engine manifold assembly tends to produce for high charges of the engine an excessive or insufficient fuel/air ratio.
In such a carburetor, the direct action of the pulses at the opening of the jet system is counterbalanced by an indirect action exerted on the emulsion well and which may be adapted to the characteristics of the particular carburetor-manifold assembly.
In a first embodiment of the invention, the movable wall constitutes a partition separating a capacity into said chamber communicating with the emulsion well and an additional chamber connected to the venturi of the carburetor.
In another embodiment, said chamber constitutes a damping capacity located above said fuel emulsion well and communicating with an emulsion tube placed in the emulsion well through a calibrated restrictor, said capacity communicating with the air inlet of the carburetor through a calibrated passage and being separated from the atmosphere by said movable wall.
The invention will be better understood from the description which follows of down-draught carburetors fitted with correction devices and constituting particular embodiments of the invention, given by way of examples.